This invention relates to a vibration damper for use in a wheel brake for suppressing brake squeals during braking.
The braking pads vibrate and resonate when they are brought into frictional contact with its rotor to brake the wheel. Such resonance causes brake squeals. Unexamined Japanese patent publications 4-54324 and 4-54325 disclose vibration dampers for damping such brake squeals in an active manner.
The former publication discloses a vibration damper including a piezoelectric element as a vibrator for applying vibrations having a frequency out of the audible range to friction members (pads), and another vibration damper comprising a piezoelectric element as a vibration detector for detecting vibrations of the friction members, and another piezoelectric element for applying vibrations of inaudible frequencies to the friction members when the vibration detector detects vibrations of the friction members.
The vibration damper disclosed in the latter publication comprises a piezoelectric element as a vibration detector for detecting vibrations of the friction members, and a vibrator for applying vibrations which act to damp the vibrations detected by the vibration detector. In one embodiment, the vibrator applies vibrations having a 180.degree. phase difference from the vibrations detected. In another embodiment, two piezoelectric elements are used.
It is generally believed that brake squeals have a frequency range of 1.5-12 kHz. On the other hand, the frequency range of sounds audible to humans has its upper limit at around 16-20 kHz and its lower limit at around 16-20 Hz. This means that brake squeals are audible to humans. Thus, the vibration dampers disclosed in the former publication, which are used to damp vibrations out of the audible frequency range, are useless as dampers for suppressing audible brake squeals.
In the vibration dampers disclosed in the latter publication, if the settings for the elements of the damper remain unchanged, the control circuit can not always perform its optimum vibration damping function due to inevitable changes, such as wear and quality change in material, of the pads with the lapse of time.
More specifically, control constants are set for the respective control elements of the control unit of a vibration damper when the damper is first put into use. In a conventional arrangement, it was impossible to adjust these constants even if the brake pads deteriorate with time. If, due to such age-related deterioration of the pads, the phase of the vibrations applied to the pads by the vibrator shifts 180.degree., such vibrations will increase, instead of reducing, vibrations of the pads.
In the arrangement of the latter publication, the vibration detector detects all the vibrations and does not judge whether or not the vibrations detected have a frequency that can cause brake squeals. Thus, the control circuit will be activated even when the detector detects vibrations having frequencies not within the squeak-causing frequency range, thereby unnecessarily vibrating the friction members with the vibrator means. This quickens wear of the friction members.
An object of this invention is to provide a vibration damper which makes it possible to adjust the control circuit if the control constants for the brake friction members change due to deterioration with time of the friction members, and thus to damp brake vibrations in an optimal manner.